Dillip Kumar Chand

A set of Pd ₂ L ₄ -type isomeric coordination cages crafted with regioisomeric ligands exhibited differential behaviours. One of the isomeric cages reorganized, forming an unusual triply interlocked (Pd ₂ L ₄ ) ₂ -type cage under optimized conditions.

An ensemble of cis-Pd(tmeda)(NO3)2, L1·2PF6, L2, L3·3PF6, and L4 in a 5:1:1:1:1 ratio in a water medium resulted in a pair of coexisting mixed-ligated coordination architectures [Pd2(tmeda)2(L1)(L2)]6+, [1]6+, and [Pd3(tmeda)3(L3)(L4)]9+, [2]9+, exclusively. The ligands L12+ and L2 were bis-monodentate, whereas ligands L33+ and L4 were tris-monodentate. The 2-dimensional D-shaped, [1]6+, and 3-dimensional dome-shaped, [2]9+, were also prepared separately. Such architectures may interlock to form a corresponding 2-catenane or related interlocked structure, but the positive charge crafted in the ligand backbones inhibits interlocking. Although [1]6+ (host H) has an exposed cavity, [2]9+ (host H′) possesses a confined cavity. Host H did not bind nonpolar aromatic guests because of the exposed cavity, whereas host H′ could bind such guests, but of restricted size, because of the confinement. Both hosts are capable of binding polar (neutral or anionic) guests, but larger-sized polar guests are not accommodated by the confined cavity of H′. Building upon the guest-binding behavior, we demonstrated a supramolecular matchmaking phenomenon, where the coexisting self-assembled hosts H and H′ picked the larger polar (anionic) guest G and smaller nonpolar guest G′, respectively.

Complexation of Pd(II) with a symmetrical bis-monodentate ligand produces a cage or dynamic equilibrium of more than one cage of the Pd_mL_2m formulation. However, orientational isomers are expected when ligand is unsymmetrical. Dynamic equilibrium of Pd₂L₄- and Pd₃L₆-type endohedrally low-symmetry cages containing orientational isomers (13 possibilities) were obtained from Pd(II) and pyridine-3-yl appended unsymmetrical ligand L1 having amide-phenylene-ester spacer. Crafting geometric constraint (unequal-sized coordination-arms) in ligand should bring isomeric selectivity but with altered cage-shape/size. To retain cage-shape/size, a steric constraint was implanted by peripheral substitution on L1. Quinolin-3-yl was placed instead of pyridine-3-yl at the amide-end and ester-end of L1 to obtain regioisomeric ligands L2 and L3, respectively. Solvent-dependent nuclearity control (Pd₂L₄ in CH₃CN; Pd₃L₆ in dimethyl sulfoxide (DMSO)) and substituent-driven isomeric control (all-cis(2,2) isomer) have been obtained by mixing Pd(II) with L2 or L3. The obtained all-cis(2,2)-Pd₃L₆ of L2 and L3 are designated as isomer-1 and isomer-2, under ligand isomerism. The isomer-1 in DMSO could be switched to Pd₂L₄ form by adding CH₃CN and reverted to Pd₃L₆ form by evaporation of CH₃CN. However, the isomer-2 in DMSO showed insignificant changes upon addition of CH₃CN. Thus, we demonstrated differential switching modes of a pair of isomeric cages that are endohedrally low-symmetry.

Conformational adaptability of a converging ligand, rigidity of a family of complementary diverging ligands, and Pd( ii ) has been utilized to achieve an unprecedented 3-fold heteromeric completive self-sorting.

The binding of pyrazine N , N ′-dioxide as a guest using three isomeric double-cavity coordination cages of Pd ₃ L ₄ formulation revealed the emergence of behaviour where a subtle difference in ligand design resulted in unexpected differences.

Encapsulation of a guest by a host forming (G-in-H) type host-guest complexes is the central theme of supramolecular chemistry. Binding of typical hosts (H_in) in the cavity of larger host (H_out) to form (H_in-in-H_out) type assemblies is also known. However, incorporation of host-guest complexes in a larger host forming [(G-in-H_in)-in-H_out] type assemblies, where the (G-in-H_in) is composed of one guest and one host is relatively unexplored. A recent report describes binding of host-guest complexes consisting of one guest and two hosts in a larger host molecule. In the present work, we have prepared a nano-sized polycationic molecular cuboid of Pd₈(L')₈(L)₄ type H_out that is capable of encapsulating two units H_in such as dibenzo[3n]crown-n (n ranges from 6 to 8). Dibenzo[24]crown-8 or dibenzo[21]crown-7 (bigger sized) is found to be selectively recognized by the molecular cuboid against dibenzo[18]crown-6 (smaller sized). The polycationic molecular cuboid could bind a family of [Cs(crown ether)₂]⁺ type cationic host-guest complexes composed of one unit of the guest (Cs⁺) and two units of the H_in (crown ether). All these self-assembled complexes are prepared in water medium where the surrogate nuclear waste like Cs⁺ is deeply located as covered by two units of crown ether, that is in turn sheltered by the molecular cuboid. All the [G-in-(H_in)₂-in-H_out] type assemblies are characterized using spectroscopic techniques and single crystal structures. Relative orientation of a pair crown ether around the Cs⁺ ion is found to be different in the native-form when compared with that of encapsulated-form in the molecular cuboid.

Understanding the key role of conformational adaptability in biological processes is crucial to mimic the remarkable applications inherent to biological systems. Motivated by the efficacy of conformational adaptability, we equipped a conformationally-adaptive ligand in low-symmetry cis-Pd2La2Lx2-type coordination cages. A family of five cis-Pd2La2Lx2-type cages was assembled by complementary ligand pairing of a conformationally adaptable converging ligand (La-type) in combination with diverging rigid ligands (Lx-type) of varied lengths. Integrative self-sorting of the individual cages showed that the converging ligand adapts to three distinct conformations in the Pd2La2Lx2-type architecture to accommodate Lx-type ligands of varying sizes. Through a series of experiments, we found a higher order, i.e., 2-fold heteromeric completive self-sorting outcomes of two co-existing Pd2La2Lx2-type cages, where two chosen complementary rigid ligands could induce any two different conformations of the converging ligand in the co-existing cages. Then we further pushed the intricacy and demonstrated the unprecedented 3-fold heteromeric completive self-sorting in coordination cage systems, where the conformationally-adaptive ligand adapts three distinct conformations in three co-existing Pd2La2Lx2-type cages. This study paves the way for the utility of conformational adaptability to achieve switchable size, shape, and functionality in supramolecular systems toward bio-relevant applications.

A family of Pd₃L₄, Pd₄L₄, and Pd₅L₄-type cages possessing two, three and four cavities, capable of encapsulating up to two, three and four units of pyrazine-N,N'-dioxide (PZDO), respectively.

The hydrophobicity of alkyl chain plays crucial role in the aggregation behavior of metallomicelle‐forming coordination complexes, in water, thereby influences the reaction rates of metallomicelle catalyzed organic reactions. Here, we have demonstrated the chain length variation for alkyl group that is covalently attached to ligand backbone of a MoO₂(L_n)(OH₂) type complex showing metallomicelle formation in water when the alkyl group is eight‐carbon chain or beyond. The impact of alkyl chain length on reactivity of organic transformation was systematically studied for oxidative coupling of benzylic alcohol with 1,2‐diaminobenzene forming benzimidazole, in water, as a model reaction. The efficiency of Mo‐complexes increases with chain length for the mentioned organic reaction performed in water medium under open air as sole oxidant without the involvement of base/additive, thus making the protocol attractive. This work develops a unique study to understand organic reactions in micellar medium as well as an efficient pathway for the direct synthesis of benzimidazoles from alcohols and diamines.

The term ligand isomerism stands for two or more isomeric coordination complexes having regioisomeric ligands coordinated around the metal center. Single‐cavity discrete coordination cages (SCDCCs) and multi‐cavity discrete coordination cages (MCDCCs) are exotic class of self‐assembled complexes that should be suitable for exploration of ligand isomerism. This work describes rare varieties of double‐cavity tetranuclear, triple‐cavity pentanuclear and quadruple‐cavity hexanuclear MCDCCs to exemplify ligand isomerism. Square planar Pd(II) and pyridine‐based bis‐, tris‐ and tetrakis‐monodentate ligands are employed as the modular building blocks for constructing the cages. The frameworks of all the ten cages studied here (four reported and six new) contain trinuclear Pd ₃ L ₆ type double‐walled triangular core (or sub‐framework) that is decorated with one, two and three units of Pd ₂ L ₄ type entity or sub‐framework resulting in tetra, penta and hexanuclear MCDCCs, respectively. Suitable incorporation of isomeric arms as part of the double‐walled trinuclear core by sourcing from the basket of regioisomeric ligands would offer ligand isomerism in the MCDCCs. Our ligand design afforded four members for the tetra or pentanuclear and two for the hexanuclear architectures to demonstrate ligand isomerism in MCDCCs.